The present invention was made from activities under a grant from the U.S. government. The Government has rights in this invention pursuant to Contract No. DE-FG36-08GO18134 awarded by the U. S. Department of Energy.
On a volume basis, alcoholic fermentation represents one of the largest fields of industrial biotechnology being used for production of traditional alcoholic beverages (wine, beer, strong alcoholic beverages, etc.) as well as industrial and fuel ethanol. The feedstock for fermentation is typically a sugar source, such as glucose derived from corn starch or sucrose from sugar beets and sugar cane, which are renewable agricultural crops. Fuel ethanol competes with, or at least functions as supplement to, conventional fossil derived fuels but lacks the environmental impact because no more carbon is emitted into the atmosphere by burning ethanol than was assimilated from the atmosphere by the growing crop. However, to compete with conventional fossil fuels and be more widely adapted, the cost of producing ethanol from renewable crops needs to be reduced and the carbon dioxide emitted by the ethanol production process itself also needs to be reduced to lower the environmental impact.
One of the most costly components of fuel grade ethanol production is the energy required for distillation of the ethanol from the aqueous fermentation broth in which it is made. Typically, the energy used for distillation is obtained from conventional fossil fuels. Therefore, any savings that can be realized from lowering the energy costs incident to distillation will simultaneously lower the cost of ethanol production and the carbon dioxide emissions incident to its production.
One of the reasons distillation costs for ethanol production is high is that water comprising the fermentation broth has one of the highest heat capacities of ethanol miscible liquids. Another reason is that ethanol and water form an azeotrope making it difficult to separate the species efficiently by distillation, especially because their boiling points are only separated by 22° C. It would reduce distillation cost if ethanol could first be extracted from the aqueous fermentation broth into an organic solvent having a lower heat than water, and which has a significantly different boiling point and does not form an azeotrope with ethanol.
Certain selectively permeable membranes have been described that permit preferential passage of organic compounds, such as ethanol relative to water. Examples of such selective membranes have been described in U.S. Pat. Nos. 7,794,593, 7,517,455, 7,105,089 and 7,122,709. It would be beneficial to the field of ethanol production, or more generally to the field of production of any organic molecule by fermentation, if an efficient system could be designed that exploits the selectively permeability of such membranes to transfer the compound from the aqueous fermentation broth into an organic solvent, which would reduce the carbon footprint, cost and energy associated with the manufacturing and separation of organic compounds made by fermentation.
The invention described hereafter provides such a system, particularly exemplified for ethanol manufacturing, but generally applicable to the manufacturing of any organic compound by fermentation where the compound can be extracted across a selectively permeable membrane to separate it from the aqueous fermentation broth.